Generally, a derailleur for the bicycle comprises a fixing member and a derailleur body provided with; a four link pallalelogram mechanism comprising a support member, two linkage members, and a movable member having a chain guide; and a return spring biasing the movable member with respect to the support member. A control wire is operated to transform the link mechanism to reciprocate the chain guide, so that a driving chain guided therewith is switched to a desired sprocket of a plurality of multistage sprockets for changing the bicycle speed.
The link mechanism is well-known which is supported swingably, other than fixed, to the fixing member through one transverse shaft and has a spring between the fixing member and the support member at the link mechanism. The spring tends to bias the chain guide radially outwardly of the multistage sprockets. Such a derailleur allows its body to swing with respect to the fixing member by use of the balance between the spring force and a change of chain tension caused by a transformation of the link mechanism.
Therefore, the chain guide at the aforesaid conventional derailleur moves following the derailleur body which swings with respect to the fixing member after the link mechanism is transformed. Hence, the chain, when switched, is guided by the chain guide moved by the transformation of the link mechanism and contacts with the lateral side of an adjacent larger diameter sprocket just to engage with the chain, thereby being subjected to a greater surface pressure. As a result, a resistance increases against the speed change to make operation of the wire difficult, thereby creating the problem of generating sounds and causing frequent wearing of the chain and sprockets.